In recent years, the following fields have been extensively developed because of soaring crude oil prices and the depletion of oil resources that may occur in the near future: deep oil fields that have not attracted much attention; oil fields in severe corrosion environments such as sour environments containing hydrogen sulfide and the like; and gas fields in such severe corrosion environments. Oil country tubular goods (OCTGs) used in such environments need to have properties such as high strength and excellent corrosion resistance (sour resistance).
To cope with such requirements, for example, Japanese Unexamined Patent Application Publication No. 2007-16291 discloses a low-alloy steel, having excellent resistance to sulfide stress cracking (SSC resistance), for oil well tubes. The low-alloy steel contains 0.20% to 0.35% C, 0.05% to 0.5% Si, 0.05% to 0.6% Mn, 0.8% to 3.0% Mo, 0.05% to 0.25% V, and 0.0001% to 0.005% B on a mass basis and is adjusted such that the inequality 12V+1−Mo≧0 holds. In a technique disclosed in JP '291, when Cr is further contained therein, the contents of Mn and Mo are preferably adjusted depending on the content of Cr such that the inequality Mo−(Mn+Cr)≧0 is satisfied. This allows resistance to sulfide stress cracking (SSC resistance) to be enhanced.
Apart from seamless steel tubes, Japanese Unexamined Patent Application Publication No. 06-235045 discloses an electric resistance welded steel pipe which has excellent resistance to sulfide stress corrosion cracking and which contains 0.05% to 0.35% C, 0.02% to 0.50% Si, 0.30% to 2.00% Mn, 0.0005% to 0.0080% Ca, 0.005% to 0.100% Al, and one or more of 0.1% to 2.0% Mo, 0.01% to 0.15% Nb, 0.05% to 0.30% V, 0.001% to 0.050% Ti, and 0.0003% to 0.0040% B on a mass basis. The contents of S, O, and Ca therein satisfy the inequality 1.0≦(% Ca){1−72(% O)}/1.25(% S)≦2.5 and the contents of Ca and O therein satisfy the inequality (% Ca)/(% O)≦0.55. In a technique disclosed in JP '045, since the addition of Ca leads to an improvement in sour resistance, the content of Ca is adjusted to satisfy the inequality (% Ca)/ (% O)≦0.55, whereby the molecular ratio of (CaO)m.(Al2O3)n, which is a deoxidation product, can be controlled to satisfy the inequality m/n<1; the stretching of complex inclusions in an electrically welded portion is avoided; the production of plate-like inclusions is prevented; and deterioration of SSC resistance due to hydrogen induced blister cracking originating from such plate-like inclusions can be prevented.
Japanese Unexamined Patent Application Publication No. 2000-297344 discloses an oil well steel which has excellent toughness and resistance to sulfide stress corrosion cracking and which is made of a low-alloy steel containing 0.15% to 0.3% C, 0.2% to 1.5% Cr, 0.1% to 1% Mo, 0.05% to 0.3% V, and 0.003% to 0.1% Nb on a mass basis. The sum of the contents of precipitated carbides is 1.5% to 4%. The percentage of the content of an MC-type carbide in the sum of the carbide contents is 5% to 45% and the content of a M23C6-type carbide therein is (200/t) % or less (t (mm) is the thickness of a product). The oil well steel can be produced by performing quenching and tempering at least twice.
Japanese Unexamined Patent Application Publication No. 2000-178682 discloses an oil well steel which has excellent resistance to sulfide stress corrosion cracking and which is made of a low-alloy steel containing 0.2% to 0.35% C, 0.2% to 0.7% Cr, 0.1% to 0.5% Mo, and 0.1% to 0.3% V on a mass basis. The sum of the contents of precipitated carbides is 2% to 5%. The percentage of the content of an MC-type carbide in the sum of the carbide contents is 8% to 40%. The oil well steel can be produced by performing quenching and tempering only.
Japanese Unexamined Patent Application Publication No. 2001-172739 discloses an oil well steel pipe which has excellent resistance to sulfide stress corrosion cracking and which contains 0.15% to 0.30% C, 0.1% to 1.5% Cr, 0.1% to 1.0% Mo, Ca, O (oxygen), and one or more of 0.05% or less Nb, 0.05% or less Zr, and 0.30% or less V, the sum of the contents of Ca and O being 0.008% or less, on a mass basis. Inclusions in steel have a maximum length of 80 μm or less. The number of inclusions with a size of 20 μm or less is 10 or less per 100 mm2. Such an oil well steel pipe can be produced by performing direct quenching and tempering only.
Factors affecting SSC resistance are extremely complicated and therefore conditions for allowing 110-ksi class high-strength steel pipes to stably ensure SSC resistance have not been clear. At present, OCTG (Oil Coutry Tubular Goods) which can be used as oil well pipes in severe corrosion environments and which have excellent SSC resistance cannot be manufactured by any of techniques disclosed in JP '291, JP '344, JP '682 and JP '739. A technique disclosed in JP '045 relates to an electric resistance welded steel pipe in which the corrosion resistance of an electrically welded portion may possibly be problematic in a severe corrosion environment. The steel pipe disclosed in JP '045 is problematic as an oil well pipe used in a severe corrosion environment.
It could therefore be helpful to provide a high-strength seamless steel tube with excellent resistance to sulfide stress cracking (SSC resistance). The term “excellent resistance to sulfide stress cracking (SSC resistance)” means that in the case of performing constant load testing in an aqueous solution (a test temperature of 24° C.), saturated with H2S, containing 0.5% by weight of acetic acid (CH3COOH) and 5.0% by weight of sodium chloride in accordance with regulations specified in NACE TM 0177 Method A, cracking does not occur at an applied stress equal to 85% of the yield strength for a test duration of more than 720 hours.